1. Field of the Invention
The present invention is directed to a spectroscopic imaging method.
2. Description of the Prior Art
For more than four decades, magnetic resonance spectroscopy has been used in fundamental research in physics, chemistry, and biochemistry, for example as a technique of analysis or for the structural clarification of complex molecules. Clinical magnetic resonance spectroscopy refers to magnetic resonance spectroscopy using clinical magnetic resonance tomography apparatuses. The methods of localized magnetic resonance spectroscopy are distinguished from those of magnetic resonance imaging essentially only in that in spectroscopy chemical displacement is also resolved, in addition to tomographic spatial resolution. In tomographic imaging, for example multi-spin echo methods, such as the RARE (Rapid Acquisition with Relaxation Enhancement) method are known, in which, in contrast to a single spin echo pulse sequence, additional spin echoes are produced by adding additional 180xc2x0 radio-frequency pulses. For multi-spin echo methods in tomographic imaging, improvements are known that seek to achieve a high signal level for a largest possible number of additional spin echoes. Here, reference is made to, among other sources. For example, A. A. Maudsley, xe2x80x9cModified Carr-Purcell-Meiboom-Gill Sequence for NMR Fourier Imaging Application,xe2x80x9d Journal of Magnetic Resonance 69, 1986, pp.488-491, discloses producing a 90xc2x0 phase offset from pulse to pulse for radio-frequency pulses following one another immediately in time.
Currently, in clinical applications two localization methods are dominant for magnetic resonance spectroscopy. One type includes individual volume techniques based on echo methods, in which a spectrum of a target volume selected beforehand on the basis of proton images is recorded. Another type are spectroscopic imaging methods, known as CSI methods (Chemical Shift Imaging), that simultaneously enable the recording of spectra of a multiplicity of spatially contiguous target volumes.
The single-volume techniques standardly used today are based on an acquisition of a stimulated echo or of a secondary spin echo. In both cases, a spatial resolution takes place by successive selective excitation of three orthogonal layers. The target volume is defined by the slice volume of these three layers. Only the magnetization of the target volume experiences all three selective radio-frequency pulses and thus contributes to the stimulated and secondary spin echo. The spectrum of the target volume is obtained by one-dimensional Fourier transformation of a time signal corresponding to the stimulated echo or to the secondary spin echo.
Spectroscopic imaging methods are used both in clinical phosphorus spectroscopy as well as in proton spectroscopy. A 3D CSI pulse sequence has, for example, the following steps: After a non-layer-selective 90xc2x0 radio-frequency pulse, a combination of magnetic phase-coding gradients of the three spatial directions is activated for a defined time duration, and subsequently the magnetic resonance signal is read out in the absence of all gradients. This procedure is repeated as often as necessary with different combinations of phase-coding-coding gradients until the desired spatial resolution has been achieved. A four-dimensional Fourier transformation of the magnetic resonance signals supplies the desired spatial distribution of the resonance lines. If the above-described non-selective radio-frequency pulse is replaced by a layer-selective excitation, consisting of a frequency-selective radio-frequency pulse and a corresponding magnetic gradient, one phase-coding direction can be omitted, and in a 2D CSI pulse sequence of this sort the measurement time is reduced in relation to the 3D CSI pulse sequence.
In clinical proton spectroscopy, the intensive water signals are often suppressed by means of water suppression techniques. One such technique for water suppression is, for example, the CHESS technique, in which the nuclear spins of the water molecules are first selectively excited by narrowband 90xc2x0 radio-frequency pulses, and their cross-magnetization is subsequently dephased through the switching of magnetic field gradients. For an immediately subsequent spectroscopic imaging method, in the ideal case no detectable magnetization of the water molecules is therefore available. In methods using a suppression of a dominant resonance line, however, lines adjacent to the dominant resonance line are also at least partially saturated as well, so that, disadvantageously, these lines appear only weakly, or not at all, in the associated spectrum.
In general, fast CSI methods are based on multiecho sequences. Besides the one desired echo per readout interval, secondary echoes and stimulated echoes also occur in multiecho sequences. In connection with the large offset frequencies due to the chemical displacement, this leads to the formation of two echo groups, known as an even echo family and an odd echo family.
One of the fast CSI methods is known as the CSI-U-FLARE method. Here a distinction is made between variants known as coherent, phase-cyclical, and pushing-apart. In the coherent CSI-U-FLARE method, for the suppression of the above-cited formation of two echo groups an attempt is made, inside an acquisition window, to superimpose the even and the odd echoes in phase-coherent fashion by carrying out a fine adjustment of gradients and sequence parameters. Because the above-cited superimposition succeeds only for a single resonance line, and because slight de-adjustments already cause significant artefacts, the coherent CSI-U-FLARE method has not achieved significance in spectroscopic imaging.
In the phase-cyclical CSI-U-FLARE method, two complete measurements are carried out that are distinguished from one another only in that the refocusing radio-frequency pulses have respective phase angles that are offset by, for example, 90xc2x0. By means of a corresponding subsequent processing of the two measurement results, an unambiguous identification of the two echo families can be achieved. The measurement results are thereby separated in the time domain, and for the two echo families the corresponding spectra are reconstructed and the two spectra are added, with a mirroring of the spectrum for one of the echo families being necessary before the addition. The necessary mirroring of the spectra has, for example, the result that, given an incomplete separation of the measurement results, artefacts arise in the two spectra. In relation to the theoretically ideal coherent method, in the phase-cyclical method the overall measurement time is doubled due to the two measurements, and the signal-noise ratio for a comparable measurement time is reduced to approximately 71%.
In the pushing-apart variant of the CSI-U-FLARE method, the two echo families are purposely pushed apart in such a way that either only one of the echo families is detected in an acquisition window, or both are sufficiently distant from one another that they can be acquired individually. For this purpose, gradient time surfaces are intentionally misadjusted. In relation to the theoretically ideal coherent method, the signal-to-noise ratio for a comparable measurement time is thereby reduced to 50 percent. Further explanation of the CSI-U-FLARE method can be found in the article by W. Dreher et al., xe2x80x9cImproved Proton Spectroscopic U-FLARE Imaging for the Detection of Coupled Resonances in the Rat Brain in Vivo,xe2x80x9d Magnetic Resonance Imaging, volume 17, no. 4, 1999, pp. 611-621.
A general disadvantage of the CSI-U-FLARE methods is that an excited transversal magnetization during the measurement time is partially converted into a longitudinal magnetization, and thus is not available for signal acquisition. A further disadvantage of the above-cited methods is that the amplitudes of the various echo families during the first echo first must be stabilized in order to ensure a uniform distribution of both echo families and a subsequent signal curve that decreases monotonically. Due to this, the first echo cannot be used, or can be used only in limited fashion.
An object of the present invention is to provide a rapid spectroscopic imaging method that has a high signal-to-noise ratio and that reduces the above-cited disadvantages of the known rapid CSI methods.
This object is achieved according to the invention in a spectroscopic imaging method for a magnetic resonance apparatus, containing the following method steps:
(a) Transmission of an excitation radio-frequency pulse having a flip angle of approximately 90xc2x0,
(b) Execution of the following method steps:
Transmission of a rephasing radio-frequency pulse having a flip angle of 180xc2x0.
Activating a phase-coding gradient.
Activating a readout gradient and acquisition of a magnetic resonance signal.
(c) Execution at least three times of the following method steps:
Transmission of a rephasing radio-frequency pulse having a flip angle of 180xc2x0 and having a 90xc2x0 phase offset in relation to a chronologically immediately preceding rephasing radio-frequency pulse.
Activating the readout gradient and acquisition of a magnetic resonance signal.
By the use of the 90xc2x0 phase offset from one 180xc2x0 rephasing radio-frequency pulse to the next, effects that lead to a loss of signal level of the magnetic resonance signals, caused by phase differences within an excited nuclear spin group due to the frequency differences that occur and that are to be acquired during the spectroscopy, as well as due to deviations of the flip angle from 180xc2x0, are compensated to such an extent that, despite the frequency differences, a formation of two echo families is prevented. In contrast to the coherent CSI-U-FLARE method, in which a phase-coherent superposition of even and odd echo families can be carried out only for one resonance line, here the superposition can successfully be carried out for a number of resonance lines. In addition, in contrast to the CSI-U-FLARE methods the magnetization is kept almost constantly transversal, and thus is fully available for the acquisition of magnetic resonance signals. In addition, in contrast to the CSI-U-FLARE methods a stabilization during the first echo is not required, so that all acquired magnetic resonance signals can be used for the formation of magnetic resonance spectra. The above differences result in a higher signal-noise ratio in comparison with the known fast CSI methods.
In addition, an improved signal-noise ratio can be achieved in comparison with the classical CSI methods, in which magnetic resonance signals are read out in the absence of gradients and the spatial coding takes place exclusively by means of phase-coding gradients. The reason for this is that in the inventive method a long acquisition time, on the order of magnitude of the T2 time, is available, whereas in the classical CSI method only an acquisition time on the order of magnitude of the T2* time can be used. Thus, in the inventive method an improved signal-noise ratio is achieved despite a decay of the magnetic resonance signal as a consequence of the T2 time, due to accumulation effects resulting from the long acquisition times. This is true in particular for apparatuses having a high magnetic flux density of the basic magnetic field, in which the T2* time is often very much smaller than the associated T2 time.
A further embodiment of the inventive spectroscopic imaging method contains the following additional method steps:
(d1) Multiple repetition of steps (b) and (c), with a variation of the phase-coding gradient per repetition, and
(e1) Multiple repetition of steps (a) to (d1), with a variation of at least one chronological spacing between steps (a) and (b) from one another per repetition.
The magnetic resonance signals that correspond to one another with respect to a chronological sequence within steps (b) and (c) in the repeated execution are stored in respective data sets, and the data sets are reconstructed individually for the formation of magnetic resonance spectra, for example by means of a Fourier transformation, and the reconstructed data sets are added.
Another embodiment of the inventive spectroscopic imaging method contains the following additional method steps:
(d2) Multiple repetition of step (b) with a variation of the phase-coding gradient and with a transmission of the rephasing radio-frequency pulse with a 90xc2x0 phase offset in relation to a chronologically immediately preceding rephasing radio-frequency pulse per repetition, and
(e2) Multiple repetition of steps (a) to (d2), with a variation of at least one chronological spacing between steps (a) and (b) from one another per repetition.
In another embodiment, the spectroscopic imaging method contains the following additional method steps:
(d3) Execution of steps (a) and 9b) with a multiple repetition of step (b) with a variation of the phase-coding gradient and with a transmission of the rephasing radio-frequency pulse with a 90xc2x0 phase offset in relation to a chronologically immediately preceding rephasing radio-frequency pulse per repetition.
e3) Multiple repetition of step (d3) with a variation of at least one chronological spacing between steps (a) and (b) from one another per repetition.
In the latter two embodiments for additional method steps, in relation to the first further embodiment, a shortening of the measurement time can be achieved, or in the same measurement time a larger data matrix can be recorded. For the latter two embodiments for additional method steps, a phase correction is taken into account for this purpose.
In another embodiment, from the at least four magnetic resonance signals acquired with an unmodified phase-coding gradient, correction data are determined for a phase correction, with which magnetic resonance signals that are acquired with a varied phase-coding gradient from rephasing radio-frequency pulse-to-rephasing radio-frequency pulse can be correspondingly phase-corrected. For the description of the principles of a known method for phase correction, reference is made, for example, to the article by H. Bruder et al., xe2x80x9cImage Reconstruction for Echo Planar Imaging with Nonequidistant k-Space Sampling,xe2x80x9d Magnetic Resonance in Medicine 23, 1992, pp. 311-323.
In another embodiment, the spectroscopic imaging method contains the following additional method step between steps (a) and (b):
(ab) Transmission of an initial rephasing radio-frequency pulse having a flip angle of 180xc2x0.
If this step (ab) is executed, then varying the chronological spacing between steps (a) and (b) in the aforementioned embodiments includes varying at least one of the chronological spacing between steps (a) and (ab) and between steps (ab) and (b).
In this way, in particular in connection with the variation of the chronological spacing of steps (ab) and (b) for the coding of the chemical displacement by displacing the initial rephasing radio-frequency pulse within a time interval that remains constant and that is adjoined by the excitation radio-frequency pulse and by step (b), magnetic resonance spectra with effective homonuclear decoupling can be recorded.